I have heard, 22, 26, 28, even 32 MPG from a cummins before.
I have drove.........
92 Cummins tow truck, 4x4, 4:10's, five speed. 14 best, empty.
95 Cummins 2x4, Auto, 3.55's. 17 best, no trailor, no weight....
2002 Cummins, 1 ton dually, 4x4, felt like 4:10's, it got about 18 light.
I have seen.........12-18 from them, my 71 318 powerd D200 get 14, behind a lighter loaded powerstroke getting 12 (98 powerpoke, 2x4 dually), and monkeys fly right out my butt! OK, maybe not, but still.........................over 20 is BS in my book.
Chev Duramax 1 ton 4x4 12' flatbed, got 12.3 loaded, from a honest guy.